How Do I Keep These Pants From Acting Like A Dress?
you have to fix the weights of those bones of those legs, just like i show you here...
so as can you see the green weight there, there are green areas influenced in the other leg because the other legs weight, this will always happen when you parent your armature with automatic weights.



so you have to change the paint mode to subtract and increase or decrease the strenght of the brush and then start brushing the undesired weights.




other more quicker way to do this is to enter to edit mode and go and select the vertex group that you want to fix and you will notice that both legs thighs are selected so unselect one of them (the left one in this case) and then click in the button remove and then press A to unselect everything and press the select button again to the vertex group and will see that its not anymore selected the right leg thigh. and then repeat the same process in the other bones that have wrong assigned weights, and then if you go to pose mode again you have fixed it. happy blending!!